1. Field of the Invention
This invention relates to an ink set, an ink-jet printing method, a recording unit, an ink cartridge and an ink-jet printing apparatus.
2. Related Background Art
In the course of the increasing popularity of ink-jet printers and other similar devices as we observe today, they are required to print high quality images than ever. It is well known that one of the major factors that determine the printing quality is the ink dots formed on the printing medium or the optical density (to be also referred to as OD hereinafter) of the ink dots. In the operation of printing black characters, the higher the OD value of the dots formed by means of black ink on the printing medium, the better the contrast to the base color of the printing medium and the quality of the image formed of ink dots. Similarly, the image formed of ink dots by using ink of some other color, which may be cyan, magenta or yellow, is better when the OD value of the dots is high.
The amount of the coloring material of ink that is left on the surface of the printing medium without permeating the latter is one of the various factors that affect the quality of the dots formed by ink on the printing medium. From this point of view, ink is normally ejected from the printing head at a rate as high as possible. A simple and easy way for increasing the amount of ink applied to a same area of the printing medium is to repeat the scanning operation of the printing head for a number of times so as to eject ink repeatedly onto the same area. This technique is currently very popular.
Another technique for increasing the amount of the coloring material of ink remaining on the surface of the printing medium is to apply a treatment liquid that makes the coloring material undissolvable onto the printing medium with ink in order to raise the amount of the coloring material left on the surface of the printing medium and improve the optical density of the dot image formed on the printing medium.
The applicant of the present patent application proposed a printing apparatus and a printing method realized on the basis of this technique in Japanese Patent Application Laid-Open No. 8-281930. According to the cited patent document, when printing a black image, treatment liquid is ejected onto the printing medium to make it show a predetermined pixel pattern along with the black ink that is ejected to the image area. The treatment liquid is cationic and hence shows the polarity opposite to that of the black ink that is anionic so that it makes black ink undissolvable and raises the density of the formed image. Additionally, the image density is raised further and the color tone is prevented from shifting if ink of some other color is ejected simultaneously to show the predetermined pattern.
The invention of the above cited patent document further provides the advantages that no printing head dedicated to the treatment liquid is required and the water resistance of the formed image is improved by the treatment liquid by using a printing apparatus having a simple configuration and that it can effectively prevent the phenomenon of feathering and that of bleeding from taking place.
The European patent application EP 831135 A1 discloses a similar technique. According to this patent document, pale magenta or cyan ink is ejected onto the dots of black ink with a low density to turn the black ink undissolvable by means of an ingredient of the low pale ink.
Generally, ink is selected from the viewpoint of ink ejection performance, printing characteristics including fixability and printing quality including blurring of printed images, optical reflection density and chromogenic property. Meanwhile, it is well known that there are two types of ink in terms of the coloring material contained therein. They are dye ink and pigment ink.
Pigment ink is advantageous to dye ink from the viewpoint of water-resistance and light-resistance and adapted to print characters clearly to realize high quality printing. On the other hand, pigment ink takes more time than dye ink for fixation on a printing medium and is not satisfactory in terms of abrasion resistance of the image after fixation if compared with dye ink. Additionally, each ink dot of pigment ink formed on a printing medium by a single ink ejecting operation of a nozzle is relatively small if compared with that of dye ink. The pigment contained in pigment ink can be reliably and stably dispersed in ink by means of the electric repulsion of a polymeric dispersant that outdoes the intermolecular force applied to the pigment particles of ink that are apt to aggregate. Therefore, it is preferable to add a polymeric dispersant to pigment ink at a rate that depends on the pigment content of pigment ink. Further, it is also proposed to prepare pigment ink containing a self-dispersion type pigment.
On the other hand, in the field of ink-jet printing technology, methods of applying ink and treatment liquid that reacts with the applied ink have been proposed in order to improve the quality of printed characters and images (e.g., in terms of the water resistance and the optical density (OD) of the image formed on a printing medium) by causing the applied ink and the treatment liquid to favorably react each other on the printing medium. Some of such methods have already found practical applications.
As a result of researches conducted by the inventors of the present invention for dissolving the problems specific to pigment ink, while exploiting the advantages thereof, they obtained the following findings.
When a pigment ink where the pigment is dispersed by a polymeric dispersant is applied onto a plain paper by means of an ink-jet recording method, the pigment particles contained in the ink aggregate due to the penetration into the paper and evaporation of the solvent of ink, which is typically water. Now, in terms of the behavior of the ink on the paper, the more the amount of the polymeric dispersant is contained in the ink, the stronger the aggregational force of the ink is, which shows that the aggregational force is stronger when the polymeric dispersant is contained in the ink at a higher rate. As a result, the ink dot formed on the recording medium with the ink containing a polymeric dispersant at a higher rate tends to have a smaller diameter than that formed with the ink containing it at a lower rate, provided that a predetermined volume of ink is ejected from an in-jet head, and keep a distorted profile produced as a result of the collision of the ink with the surface of the sheet of paper. Therefore, in order to obtain ink dots showing a sufficient recording density and having a dot diameter necessary for forming an image without generating white stripes, it is necessary to increase the volume of ink ejected from the ink-jet head. However, the fixation of the ink to the printing medium may take longer time and the scratch resistance of the recorded image may be reduced because of the strong aggregational force of the pigment particles around which the polymeric dispersant are absorbed.
The use of a penetrant may be conceivable to improve the ink permeability of the printing medium in order to increase the ink dot diameter and improve the fixation of the ink to the printing medium. However, the use of a penetrant can be accompanied by a problem of a degraded ink dot profile (a degraded circumferential profile of each ink dot, due to a phenomenon typically referred to as feathering) and that of permeation of ink to the rear surface of the paper (so-called back-through) that are undesirable for producing high quality recorded images. Additionally, even if the ink dot diameter is relatively increased, the OD (optical density) of the ink dot may not be raised because the coloring material of the ink can penetrate into the printing medium.
Meanwhile, ink containing a self-dispersible pigment has been proposed. With such ink, the dot diameter can be increased because the self-dispersible pigment contained in ink has aggregational force weaker than that of ordinary ink pigment that is dispersed by a dispersant when applied onto the surface of paper. However, it is not enough for producing high quality color images.
Under the above identified circumstances, the inventors of the present invention have done intensive research on developing the ink-jet recording technology to make it highly satisfactory in terms of various factors that affect the quality of the recorded image is including fixation of ink, enlargement of ink dot diameter, uniformity of density in each ink dot, and a high optical density of each ink dot. In the course of the researches, the inventors experimented a recording process where pigment ink was used solely and another recording process where pigment ink was used with treatment liquid that was highly reactive to the pigment ink and could destabilize the dispersibility of the pigment at the time of recording for the purpose of comparison. In the latter recording process, pigment ink was applied onto the printing medium immediately before or after the application of the treatment liquid onto the printing medium in such a way that the applied pigment ink was mixed with the treatment liquid on the printing medium in a liquid state.
As a result, however, it was found that some of the obtained images showed an image quality that is far from satisfactory and worse than the quality of images formed by using only pigment ink. More specifically, in the case of a combination of pigment ink containing a pigment that was dispersed in an aqueous medium by means of a polymeric dispersant and treatment liquid that reacts with such ink, the produced ink dots showed a low optical density (OD) probably because of a small area factor of the ink dots. Although why such a phenomenon occurred is not clear, the inventors presume that the aggregation of pigment in the ink was greatly accelerated by the treatment liquid on the printing medium.
The area factor may be raised to enhance the OD by increasing the shot-in quantity of pigment ink per unit area of the printing medium. Then, however, the fixation of ink can sometimes be worsened. In the case of a combination of pigment ink containing a self-dispersible pigment and a treatment liquid that reacts with such pigment ink, it showed the phenomenon of so-called exudation or haze along the circumferences the dots formed on the printing medium to blur the profiles of the dots.
FIG. 1 of the accompanying drawings schematically illustrates a dot showing such exudation or haze. As shown in FIG. 1, the phenomenon of exudation or haze 7 can be observed along the reaction zone of the pigment ink 8 that is located at the center and the surrounding treatment liquid 6.
FIGS. 2A through 2C of the accompanying drawings schematically illustrate the presumed mechanism of appearance of the phenomenon. After treatment liquid S is applied onto a printing medium P (a sheet of plain paper in particular), pigment ink Ip containing a self-dispersion type pigment but not containing any polymeric dispersant is applied onto the treatment liquid S. Then, the treatment liquid S and the pigment ink Ip react each other to start producing a reaction product 2 (see FIG. 2B). As the reaction proceeds, exudation of the reaction product appears radially from the circular dot of ink as shown in FIG. 2C to make the entire dot appear as if it were surrounded by haze Such exudation or haze is apparently recognized as feathering that is a known phenomenon to consequently degrade the quality of printing.
The inventors of the present invention believes that such exudation or haze is a phenomenon that can be explained in a manner as described below in chemical or micro-dimensional terms. The pigment ink containing no dispersant reacts with the treatment liquid at a relatively high reaction rate so that some of the dispersed pigment particles are instantaneously subjected to dispersive destruction to produce clusters of the reaction product and, at the same time, micro-particles of the reaction product. Then, as shown in FIG. 2C, the particles of the reaction product start flowing out as the permeation front SP in the printing medium expands to consequently make exudation or haze appear.
As described above, a situation unpredictable to the inventors of the present occurred when pigment ink and treatment liquid were simply combined so that it was difficult to obtain a high quality image by ink-jet recording. As a result, the inventors realized that further efforts were required for technological development in order to achieve the initial object of eliminating the drawbacks of pigment ink, while utilizing the ink-jet recording process using treatment liquid and exploiting the advantages of pigment ink.
Furthermore, as pointed out above, a higher printing speed will be required to ink-jet printers in business applications where the demand for ink-jet printers is expected to rise dramatically. One of the large problems of high speed printers is the fixation of ink on a recording medium. If fixation of ink to the recording medium is not satisfactory in a process of successively discharging printed sheets of paper from an ink-jet printer and sequentially laying them one on the other, a second sheet of paper discharged from the ink-jet printer may be laid on the first sheet of paper while the ink on the surface of the first sheet of paper is not fixed enough so that there may arise a problem that the image on the first sheet of paper becomes blurred and/or the ink on the first sheet of paper sticks to the rear surface of the second sheet of paper to consequently damage the appearance of the printed sheets of paper. On the other hand, the demand for high quality color images to be produced by an ink-jet recording method has been intensified.
Thus, the present invention is based on the technological findings of the inventors of the present invention. It is an object of the present invention to provide an ink set that can be used to produce high quality color prints by utilizing the ink-jet recording technology using pigment ink and treatment liquid.
Another object of the present invention is to provide an ink-jet printing method and an apparatus therefor that can realize high speed fixation of ink to the recording medium without sacrificing the quality of the printed image.
In an aspect of the invention, the above objects are achieved by providing an ink set comprising first, second and third inks respectively containing aqueous media and coloring materials:
said first ink containing an anionic dye as coloring material;
said second ink containing an anionic dye as coloring material and having a lower coloring material content than said first ink;
said third ink containing a first pigment, a second pigment and a polymeric dispersant for dispersing said second pigment, both said first pigment and said second pigment being contained in said third ink in a dispersed state;
said first pigment being a self-dispersible pigment having an anionic group bound to the surface directly or via another atomic group;
said second pigment being a dispersible to said aqueous medium by means of said polymeric dispersant;
said polymeric dispersant containing at least either an anionic polymeric dispersant or a nonionic polymeric dispersant, said second ink further containing a substance which destabilizes the dispersion stability of at least either the first pigment or the second pigment in said third ink when the second ink is brought into contact with said third ink in a liquid state on a printing medium.
In another aspect of the invention, there is provided a color ink-jet recording method comprising steps of:
(1) applying a first ink containing an aqueous medium and an anionic dye as coloring material onto a printing medium by an ink-jet method;
(2) applying a second ink containing an aqueous medium and an anionic dye as coloring material and having a lower coloring material content than the first ink onto the printing medium by means of an ink-jet method; and
(3) applying a third ink containing an aqueous medium, a first pigment, a second pigment and a polymeric dispersant for dispersing said second pigment, both said first pigment and said second pigment being contained in a dispersed state, onto the printing medium by an ink-jet method;
said first pigment being a self-dispersible pigment having an anionic group bound to the surface directly or via another atomic group;
said second pigment being dispersible to said aqueous medium by means of said polymeric dispersant;
said polymeric dispersant being at least either an anionic polymeric dispersant or a nonionic polymeric dispersant;
said second ink further containing a substance which destabilizes the dispersion stability of at least either the first pigment or the second pigment in said third ink when the second ink is brought into contact with said third ink in a liquid state on the printing medium;
said step (2) being carried out either subsequent to or practically simultaneously with said step (3) so as to make the second ink and the third ink contact each other in a liquid state on the printing medium.
In the above aspects of the invention, it is possible to provide a high quality image showing a high OD value and a good edge sharpness of the ink dot. Additionally, the obtained image is excellent in terms of scratch resistance, fixation and various other respects. While the reason why the application of the second ink containing a substance which destabilizes the dispersion stability of at least either the first pigment or the second pigment subsequently to or practically simultaneously with the application of the third ink containing the first pigment and the second pigment gives such an effect is not clear, the inventors of the present invention have confirmed the following facts as a result of a series of experiments carried out for the purpose of the present invention. As the third ink containing the first and second pigments is applied onto a printing medium, a dot of ink is formed on the surface of the printing medium P with a given diameter as shown in FIG. 3A of the accompanying drawings. The size (diameter d1) of the dot is greater than that (diameter d2) of a dot of conventional pigment ink (which may be ink where a pigment is dispersed by a polymeric dispersant or ink containing a self-dispersible pigment) (d1 greater than d2). While the reason why such a phenomenon is observable is not clear, the inventors presume that the following process proceeds there. The second pigment absorbing the polymeric dispersant and the first pigment electrically repulse each other so that the aggregational force of the pigments of the third ink is made weaker than that of ink containing only a pigment being dispersed by a polymeric dispersant. As an image is printed on the surface of the paper by means of the third ink, it is difficult for the coloring materials in the ink to penetrate into the sheet of paper in a direction perpendicular to the paper because the polymeric dispersant is adsorbed to the second pigment. Concerning along the surface of the paper, the ink containing only the second pigment and the polymeric dispersant would aggregate strongly because polymer molecules become rapidly intertwined with each other in the ink due to the penetration into the paper and/or evaporation of the solvent of the ink because molecules of the pigment are mostly cross-linked by polymer molecules. On the other hand, the first pigment contained in the third ink according to the invention prevents and suppresses the intervolution and/or the cross-linking and the inter-molecular force of the pigments in the ink is reduced by the mutual repulsion of the first pigment and the polymeric dispersant so that consequently the applied ink can easily be spread along the surface of the sheet of paper and the spread is not disorderly because the aggregational force of the pigments is reduced but still there to influence the spread of ink.
Then, as the second ink containing a substance that destabilizes the dispersion stability of at least either the first pigment or the second pigment is applied to the dot of the ink that has been uniformly spread on the surface of the printing medium (see FIGS. 2A and 2C of the accompanying drawings) the substance in the second ink reacts with at least either the first pigment or the second pigmentin the third ink at the interface of the second ink and the third ink. However, since the dot of the third ink has been expanded to a large extent as pointed out above, the second ink reacts with the third ink over a large area. Additionally, since the height (t1) of the ink dot is reduced if compared with that (t2) of a comparable dot of conventional ink, the reaction between the third ink and the second ink will be completed in a short period of time. As a result, there is a remarkable improvement in the image of the dot of the ink set according to the invention in terms of fixing time and the level of fixation and also in the edge sharpness of the ink dot. Thus, it will be appreciated that effects of the present invention attributable to the above process is specific to the system where the third ink is applied onto the printing medium before or practically simultaneously with the second ink.
The fixing property and the edge sharpness of the ink dot are further improved when the second ink is made to be able to penetrate into the printing medium at an enhanced rate. This is probably because the solvent containing water is made to permeate well into the printing medium by the permeation effect of the second ink, while the third ink and the second ink are reacting each other on the surface of the printing medium. Generally, when a coloring material is made to permeate into a printing medium, the optical density of the coloring material is reduced. However, when the third ink is applied before the application of the second ink in a manner as described above for the purpose of the invention, the pigments contained in the third ink would not permeate into the printing medium so as to reduce the OD value. Rather, the coloring materials in the third ink would remain on and near the surface of the printing medium as a result of the reaction with the substance in the second ink so that the OD of the coloring materials of the third ink is improved if compared with the case where the second ink is not used.
In still another aspect of the invention, there is also provided an ink-jet recording method comprising steps of:
(1) applying a first ink containing an aqueous medium and an anionic dye as coloring material onto a printing medium by an ink-jet method;
(2) applying a second ink containing an aqueous medium and an anionic dye as coloring material and having a lower coloring material content than the first ink onto the printing medium by means of an ink-jet method; and
(3) applying a third ink containing an aqueous medium, a first pigment, a second pigment and a polymeric dispersant for dispersing said second pigment, both said first pigment and said second pigment being contained in a dispersed state, onto the printing medium by an ink-jet method;
said first pigment being a self-dispersible pigment having at least an anionic group bound to the surface directly or via another atomic group;
said second pigment being dispersible to said aqueous medium by means of said polymeric dispersant;
said polymeric dispersant containing at least either an anionic polymeric dispersant or an nonionic polymeric dispersant;
said second ink further containing a substance which destabilizes the dispersion stability of at least either the first pigment or the second pigment in said third ink when the second ink is brought into contact with said third ink in a liquid state on the printing medium;
said step (3) being carried out either subsequent to or practically simultaneously with said step (2) so as to make the second ink and the third ink contact each other in a liquid state on the printing medium.
The above described methods according to the invention provides a high quality image showing a high OD value and significantly free from haze. Additionally, the obtained image is excellent in terms of scratch resistance, fixation and various other respects. While the reasons of the above advantages of the methods according to the invention are not clear, the inventors of the present invention have confirmed the following facts as a result of a series of experiments carried out for the purpose of the present invention. Firstly, when the third ink is applied to an area of a printing medium where the second ink containing a material which destabilizes the dispersion stability of at least either the first pigment or the second pigment has been applied in such a way that the two inks are laid one on the other in a liquid state or held in contact with each other, the ink dots in the area where the second ink has been applied expand considerably to produce ink dots showing a large diameter. Thus, presumably, the effect of aggregation of the second ink and at least either the first pigment or the second pigment in the third ink is alleviated as a result of the coexistence of the first pigment or the second pigment, whichever appropriate, in the third ink and the second ink containing the substance which destabilizes the dispersion stability of at least either the first pigment or the second pigment.
More specifically, the aggregation caused by the strong reaction of the third ink and the second ink containing a substance which destabilizes the dispersion stability of at least either the first pigment or the second pigment is alleviated by the weaker reaction of the other pigment and the second ink and additionally the strong intermolecular force of the second pigment in the reactive solution is alleviated by the existence of the first pigment. The tendency of ink of spread transversally on a paper observed in a number of experiments may be attributable to the above fact.
Still additionally, since the first pigment tends to aggregate more strongly than the second pigment, haze is practically not observable after the reaction of the second ink and the first pigment when the second ink contains a substance that destabilizes the dispersion stability of the second pigment so that the edge sharpness is improved even when the dot diameter is increased.
The ink shows an excellent fixing property since a large dot diameter can be realized with a small amount of ink as pointed out above, and the first pigment makes it possible to add the polymeric dispersant to the third ink at a reduced rate to ink.
The fixing property and the dot diameter are further improved when the second ink having a high permeability rate. The treatment liquid will form a sort of ink receiving layer on the printing medium since it permeates and is spread on the printing medium. The ink receiving layer makes the ink which is subsequently applied penetrate into and spread on the printing medium quickly while reacting with each other. Consequently, a larger ink dot will be formed on the printing medium quickly.
Preferably, according to the invention, the second ink is optimized in terms of the concentration of the substance which destabilizes the dispersion stability of at least either the first pigment or the second pigment in order to further improve the image quality. The OD of the ink dot will be sufficiently high when the concentration of the substance which destabilizes the dispersion stability of at least either the first pigment or the second pigment is about ⅓ of that of the pigment in the third ink so that the former does not need to be raised excessively. The fixation of the third ink is also optimized when the concentration of the substance adapted to destabilize the dispersion stability of at least either the first pigment or the second pigment is lower than the concentration of the pigment.
Additionally, the second ink that operates as treatment liquid is applied at a rate of xe2x85x9 to xc2xd of the rate of application of the third ink to improve the OD and the edge sharpness of the formed image.
Thus, it is possible to form a high quality image showing a high OD value and an excellent fixing property with a short fixing time on a printing medium without giving rise to any haze by laying the second ink on or under the third ink so as to make them to be held in contact with each other in order to realize the above process.
For instance, an image showing a wide color reproduction range and giving little granular impression over the entire gradation can be obtained by making the second ink that operates as treatment liquid for the purpose of the invention (and containing an anionic dye having the same tonic property as the first ink and a substance adapted to destabilize the dispersion stability of at least either the first pigment or the second pigment) paler than the first color ink and combining the first and the second ink with the third pigment ink. Then, the image quality will be further improved by using a dye same as that of the first ink in the second ink. Since the dye concentration of the second ink is low, the asociation of the dye and the substance adapted to destabilize the dispersion stability of at least either the first pigment or the second pigment is little so that the second ink will remain stable. Further a high quality ink-jet image can be obtained when the third ink is black ink because the image quality of the black image is improved.
In still another aspect of the invention, there is provided a recording unit for ink-jet recording using the inks of an ink set according to the invention, said recording unit comprising an ink container containing the respective inks and a head section for ejecting the respective inks.
In still another aspect of the invention, there is also provided an ink cartridge comprising an ink container containing the respective inks of an ink set according to the invention.
In a further aspect of the invention, there is also provided an ink-jet printing apparatus comprising an ink container containing the respective inks of an ink set according to the invention and a recording head for ejecting the respective inks.